A multi-drop bus is a connection technology for mutual communication between a number of modules which share one transmission line (bus). In many cases, a processor and a memory module are connected through a multi-drop bus in a data processing device, for example. As compared to the connection technology for one-to-one communication using a number of dedicated transmission lines, a multi-drop bus has such an advantage that the number of transmission lines can be reduced.
A multi-drop bus, however, has such a disadvantage that the transfer speed cannot be increased because the characteristic impedance discontinues at the branch points of a transmission line. When the characteristic impedance discontinues, part of the signal is reflected, which causes distortion of the signal that has passed through. In addition, the input capacitance of the module attached to a small transmission line (stub) at the end of a branch lowers the impedance of the branch point, which causes signal distortion, and thus, further delays the signal propagation along the bus.
When a connector is inserted into a stub so that a module can be exchanged or added, a component of the capacitance or the inductance that is parasitic to the connector is added to the branch point, which causes the reflection or distortion of the signal that propagates along the bus. Furthermore, the reflection or distortion of the signal causes interference between symbols, which causes a bit error.
Thus, signal reflection or distortion is generated at each branch point in a multi-drop bus, and therefore, the transfer speed cannot be increased as compared to one-to-one communication. In many cases, however, the data transfer between a processor and a memory module determines the performance of the system, and thus, the demand on a multi-drop bus with increased speed is very high.
In order to solve such a problem, it has been proposed to branch the signal without greatly changing the impedance by using a directional coupler. In this case, the input capacitance of a module attached to a stub and the parasitic component of the connector can be separated from the bus, and therefore, there is such an advantage that no reflection or distortion is generated in the signal that propagates along the bus.
It has been proposed to form a multi-drop bus by using a directional coupler where two parallel lines formed as printed wires on the same board are sandwiched between two grounded conductive plates (see Patent Document 1). In this case, the impedance of the directional coupler is the same for all the directional couplers, which means a single impedance.
Alternatively, it has been proposed to form directional couplers either on the main board or a child board (see Patent Document 2). In the case where couplers are formed on the main board, the signal is discarded by the terminal resistor after being branched by a coupler even when no child board is mounted. In the case where couplers are formed on a child board, the bus formed on the child board allows the signal to repeatedly leave the main board through the connector, pass through a coupler on the child board, and return to the main board, and finally, the bus is provided with matched termination. Accordingly, such a problem arises that a connector is inserted into the bus and the signal along the bus is reflected and distorted by the parasitic effects of the connector.
It has also been proposed to apply a multi-drop bus using such directional couplers to a memory module (see Patent Document 3). In this case, the directional couplers are formed on the same board with a single impedance.
In order to solve the problem of impedance mismatch in such directional couplers, wave form shaping by using a reflected wave generated from the mismatch of the impedance has also been proposed (see Patent Document 4). In this case, the reflected wave is created on the transmission side so that distortion is added to the transmission signal, and as a result, reflection and distortion are offset in a coupled portion. In this proposal as well, the directional couplers are formed on the same board.
In addition, it has been proposed that each directional coupler have a different coupling degree in order to divide the signal so that the propagating signal amount to each module is approximately equal in the directional coupling-type bus system (see Patent Document 5). In this case as well, the directional couplers are formed on the same board with a single impedance.
Furthermore, it has been proposed to wire the bus on the mother board and the stubs on the memory modules in zigzag form so that two wires cross each other when projected so as to form a directional coupler when the memory modules are mounted on the mother board (see Patent Document 6 and Non-Patent Document 1). In this case, coupling is provided in a portion where two wires cross each other, and therefore, the crossing form is not different even when the two wires positionally shift slightly, and thus, there is such an advantage that the coupling characteristics do not change even when the position of the wires shifts. In this case as well, the directional couplers are formed with a single impedance.